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Reflection Overview

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Rapid overview

Reflection Overview

TL;DR

🔍 Interview Angle: Be ready to explain how ActivatorUtilities or JsonSerializer leverage reflection and how source generators reduce reflection cost in .NET 5+.

How it works

  • Concept: Inspect metadata and interact with types at runtime via System.Reflection.
  • Use Cases: Plugin discovery, serialization, mapping attributes to behavior, building DI containers.
  • Cost: Reflection is slower—cache PropertyInfo/MethodInfo or emit delegates (CreateDelegate) for hot paths.
var assembly = Assembly.Load("Trading.Services");
var handlers = assembly.GetTypes()
    .Where(t => typeof(ICommandHandler).IsAssignableFrom(t) && !t.IsAbstract);

foreach (var handler in handlers)
{
    Console.WriteLine($"Discovered handler: {handler.Name}");
}

Quick recall Q&A

Q: When is reflection appropriate despite its cost?

For dynamic scenarios like plugin discovery, serialization, attribute-driven behavior, or tooling where compile-time knowledge is limited. Use it outside hot paths or cache results.

Q: How do you mitigate reflection performance penalties?

Cache PropertyInfo/MethodInfo, create delegates via CreateDelegate, or use Expression trees to generate accessors. Source generators can precompute metadata to avoid runtime reflection entirely.

Q: What are alternatives to reflection for DI?

Compile-time registration, source generators, or manual wiring. Reflection simplifies auto-discovery but can slow startup; balance convenience with performance.

Q: How does reflection interact with trimming/AOT?

Trimming can remove unused members. Reflection needs preserved metadata, so mark types with DynamicallyAccessedMembers or Preserve attributes when building self-contained apps.

Q: How do you secure reflection usage?

Validate assembly paths, restrict loaded types/namespaces, and avoid executing untrusted code. Reflection can bypass encapsulation, so enforce security at the host/application level.

Q: Can reflection access private members?

Yes via BindingFlags.NonPublic, but it should be used sparingly (e.g., for testing). It can break encapsulation and may fail under IL trimming.

Q: How do you use reflection emit or System.Reflection.Emit?

To generate types/methods at runtime (dynamic proxies, serialization). It’s powerful but complex; prefer Expression trees or source generators when possible.

Q: What’s the cost of Activator.CreateInstance vs new?

Activator.CreateInstance is slower because it uses reflection. Cache constructors via compiled delegates when instantiating frequently.

Q: How does JsonSerializer leverage reflection?

It inspects types at runtime to discover properties/attributes. In .NET 6+, source generators can precompute serializers to avoid reflection overhead and enable trimming.

Q: How do you test reflection-heavy code?

Write unit tests for discovery logic (ensuring correct types are found) and integration tests that verify attributes/config drive expected behavior. Mock metadata where possible.

See also